Cold seal cohesives are generally natural or synthetic rubber latex compounds which when applied to the cold seal receptive surface of a flexible packaging substrate allow the package to be cohesively sealed by pressure and adequate dwell time at ambient temperatures. By "cold seal" herein is meant the sealing or bonding of two film layers to each other at a temperature of less than about 150.degree. F. Cold seal latexes are particularly useful in wrapping products which are heat sensitive, such as confectioneries. Cold seal cohesives are generally applied to the inside (cold seal receptive surface) of a film lamination. They may also be applied to a single web film provided that the surface is previously primed or printed. In each case, the outer film surface (facing away from the cold seal) must repel adhesion to the cold seal on the inner web in order that the roll stock may be unwound when it is used to wrap the product. In the case of the single web film, the surface opposing the cold seal is typically coated with an overlacquer (polyamide type) in order to provide sufficient release from the cold seal, i.e., prevent roll blocking. In the case of lamination, the outer web film is generally modified with migratory additives which when sufficiently bloomed to the surface promote low co-efficient of friction (C.O.F.) for machinability and cold seal release (C.S.R.). It is known from the art that certain migratory additives of a laminate film, particularly amides, coming in intimate contact with the cold seal surface would into a roll in sufficient quantities, will cause the cold seal cohesive strength to become significantly weakened. This phenomenon is known in the trade as cold seal deadening.
Biaxially oriented polypropylene (B.O.P.P.) film by itself does not give adequate cold seal release (C.S.R.) or coefficient of friction (C.O.F.) and requires the addition of additives to accomplish these objectives. These additives, which are predominantly migratory, have two main problems: (1) they must bloom to the film's surface and remain there in order to be consistently effective; and (2) they have a tendency to retard the cold seal cohesive strength. The current industry standard film for this application is a mono-layer B.O.P.P. (homopolymer amide modified) film from Hercules, Inc. designated B523 or B522. In the case of B523, the slip modified film is corona discharge treated to an equal level on both surfaces and, as such, can be used on either side for printing and laminating. In the case of B522, the film is corona discharge treated on only one side and therefore can only be printed and laminated on that surface. The untreated surface is the preferred surface for cold seal release. These films, however, need to be aged for a period of time after production at a certain temperature so that the cold seal release and C.O.F. additives sufficiently bloom to the surface for the film to be functional.
The main requirements for a monoaxially oriented or a B.O.P.P. cold seal release film include: (a) good cold seal release; (b) good anti-blocking to reverse printed inks; (c) good lamination bonds to inks and adhesives on the non-release surface; (d) good and stable optics (i.e., haze/gloss); (e) good and stable C.O.F.; and (f) minimal/no effect on cold seal cohesiveness in the finished structure.
U.S. Pat. No. 4,230,767 issued Oct. 28, 1980 to Isaka, et al. teaches a multi-layer propylene polymer film for sealing packages. However, the Isaka, et al. laminated film is a heat sealable material and Isaka does not teach cold seal release properties.
It is therefore desirable to have a B.O.P.P. film for packaging purposes which is useable in cold sealable applications and which exhibits the requirements described above.